Glycerin strengthens the bonds of molecules in bubbles so that they can stretch further. Bubbles generally do not last long and will burst immediately when they come in contact with another object.
Bubbles are made of a very thin layer of fluid that forms a sphere around a pocket of air. A bubble made from pure liquid, such as water, is not very stable and will be hard to form. Adding soap or glycerin allows the bubble to become more stable and last longer. Glycerin is thick, clear and odorless, which helps the sphere of the bubble to be thicker.